Internet Engineering Task Force (IETF) J. Touch
Request for Comments: 5925 USC/ISI
Obsoletes: 2385 A. Mankin
Category: Standards Track Johns Hopkins Univ.
ISSN: 2070-1721 R. Bonica
Juniper Networks
June 2010
The TCP Authentication Option
Abstract
This document specifies the TCP Authentication Option (TCP-AO), which
obsoletes the TCP MD5 Signature option of RFC 2385 (TCP MD5). TCP-AO
specifies the use of stronger Message Authentication Codes (MACs),
protects against replays even for long-lived TCP connections, and
provides more details on the association of security with TCP
connections than TCP MD5. TCP-AO is compatible with either a static
Master Key Tuple (MKT) configuration or an external, out-of-band MKT
management mechanism; in either case, TCP-AO also protects
connections when using the same MKT across repeated instances of a
connection, using traffic keys derived from the MKT, and coordinates
MKT changes between endpoints. The result is intended to support
current infrastructure uses of TCP MD5, such as to protect long-lived
connections (as used, e.g., in BGP and LDP), and to support a larger
set of MACs with minimal other system and operational changes. TCP-
AO uses a different option identifier than TCP MD5, even though TCP-
AO and TCP MD5 are never permitted to be used simultaneously. TCP-AO
supports IPv6, and is fully compatible with the proposed requirements
for the replacement of TCP MD5.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5925.
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RFC 5925 The TCP Authentication Option June 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Touch, et al. Standards Track [Page 2]

RFC 5925 The TCP Authentication Option June 20101. Introduction
The TCP MD5 Signature (TCP MD5) is a TCP option that authenticates
TCP segments, including the TCP IPv4 pseudoheader, TCP header, and
TCP data. It was developed to protect BGP sessions from spoofed TCP
segments, which could affect BGP data or the robustness of the TCP
connection itself [RFC2385][RFC4953].
There have been many recent concerns about TCP MD5. Its use of a
simple keyed hash for authentication is problematic because there
have been escalating attacks on the algorithm itself [Wa05]. TCP MD5
also lacks both key-management and algorithm agility. This document
adds the latter, and provides a simple key coordination mechanism
giving the ability to move from one key to another within the same
connection. It does not however provide for complete cryptographic
key management to be handled in band of TCP, because TCP SYN segments
lack sufficient remaining space to handle such a negotiation (see
Section 7.6). This document obsoletes the TCP MD5 option with a more
general TCP Authentication Option (TCP-AO). This new option supports
the use of other, stronger hash functions, provides replay protection
for long-lived connections and across repeated instances of a single
connection, coordinates key changes between endpoints, and provides a
more explicit recommendation for external key management. The result
is compatible with IPv6, and is fully compatible with proposed
requirements for a replacement for TCP MD5 [Ed07].
TCP-AO obsoletes TCP MD5, although a particular implementation may
support both mechanisms for backward compatibility. For a given
connection, only one can be in use. TCP MD5-protected connections
cannot be migrated to TCP-AO because TCP MD5 does not support any
changes to a connection's security algorithm once established.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lowercase uses of these words are not to be
interpreted as carrying RFC 2119 significance.
In this document, the characters ">>" preceeding an indented line(s)
indicates a compliance requirement statement using the key words
listed above. This convention aids reviewers in quickly identifying
or finding the explicit compliance requirements of this RFC.
Touch, et al. Standards Track [Page 4]

RFC 5925 The TCP Authentication Option June 20101.2. Applicability Statement
TCP-AO is intended to support current uses of TCP MD5, such as to
protect long-lived connections for routing protocols, such as BGP and
LDP. It is also intended to provide similar protection to any long-
lived TCP connection, as might be used between proxy caches, for
example, and is not designed solely or primarily for routing protocol
uses.
TCP-AO is intended to replace (and thus obsolete) the use of TCP MD5.
TCP-AO enhances the capabilities of TCP MD5 as summarized in Section1.3. This document recommends overall that:
>> TCP implementations that support TCP MD5 MUST support TCP-AO.
>> TCP-AO SHOULD be implemented where the protection afforded by TCP
authentication is needed, because either IPsec is not supported or
TCP-AO's particular properties are needed (e.g., per-connection
keys).
>> TCP-AO MAY be implemented elsewhere.
TCP-AO is not intended to replace the use of the IPsec suite (IPsec
and Internet Key Exchange Protocol (IKE)) to protect TCP connections
[RFC4301][RFC4306]. Specific differences are noted in Section 1.3.
In fact, we recommend the use of IPsec and IKE, especially where
IKE's level of existing support for parameter negotiation, session
key negotiation, or rekeying are desired. TCP-AO is intended for use
only where the IPsec suite would not be feasible, e.g., as has been
suggested is the case to support some routing protocols [RFC4953], or
in cases where keys need to be tightly coordinated with individual
transport sessions [Ed07].
TCP-AO is not intended to replace the use of Transport Layer Security
(TLS) [RFC5246], Secure BGP (sBGP) or Secure Origin BGP (soBGP)
[Le09], or any other mechanisms that protect only the TCP data
stream. TCP-AO protects the transport layer, preventing attacks from
disabling the TCP connection itself [RFC4953]. Data stream
mechanisms protect only the contents of the TCP segments, and can be
disrupted when the connection is affected. Some of these data
protection protocols -- notably TLS -- offer a richer set of key
management and authentication mechanisms than TCP-AO, and thus
protect the data stream in a different way. TCP-AO may be used
together with these data stream protections to complement each
other's strengths.
Touch, et al. Standards Track [Page 5]

RFC 5925 The TCP Authentication Option June 20101.3. Executive Summary
This document replaces TCP MD5 as follows [RFC2385]:
o TCP-AO uses a separate option Kind (29).
o TCP-AO allows TCP MD5 to continue to be used concurrently for
legacy connections.
o TCP-AO replaces TCP MD5's single MAC algorithm with MACs specified
in a separate document and can be extended to include other MACs.
o TCP-AO allows rekeying during a TCP connection, assuming that an
out-of-band protocol or manual mechanism provides the new keys.
The option includes a 'key ID', which allows the efficient
concurrent use of multiple keys, and a key coordination mechanism
using a 'receive next key ID' manages the key change within a
connection. Note that TCP MD5 does not preclude rekeying during a
connection, but does not require its support either. Further,
TCP-AO supports key changes with zero segment loss, whereas key
changes in TCP MD5 can lose segments in transit during the
changeover or require trying multiple keys on each received
segment during key use overlap because it lacks an explicit key
ID. Although TCP recovers lost segments through retransmission,
loss can have a substantial impact on performance.
o TCP-AO provides automatic replay protection for long-lived
connections using sequence number extensions.
o TCP-AO ensures per-connection traffic keys as unique as the TCP
connection itself, using TCP's Initial Sequence Numbers (ISNs) for
differentiation, even when static master key tuples are used
across repeated instances of connections on a single socket pair.
o TCP-AO specifies the details of how this option interacts with
TCP's states, event processing, and user interface.
o TCP-AO is 2 bytes shorter than TCP MD5 (16 bytes overall, rather
than 18) in the initially specified default case (using a 96-bit
MAC).
TCP-AO differs from an IPsec/IKE solution as follows
[RFC4301][RFC4306]:
o TCP-AO does not support dynamic parameter negotiation.
Touch, et al. Standards Track [Page 6]

RFC 5925 The TCP Authentication Option June 2010
o TCP-AO includes TCP's socket pair (source address, destination
address, source port, destination port) as a security parameter
index (together with the KeyID), rather than using a separate
field as an index (IPsec's Security Parameter Index (SPI)).
o TCP-AO forces a change of computed MACs when a connection
restarts, even when reusing a TCP socket pair (IP addresses and
port numbers) [Ed07].
o TCP-AO does not support encryption.
o TCP-AO does not authenticate ICMP messages (some ICMP messages may
be authenticated when using IPsec, depending on the
configuration).
2. The TCP Authentication Option
The TCP Authentication Option (TCP-AO) uses a TCP option Kind value
of 29. The following sections describe TCP-AO and provide a review
of TCP MD5 for comparison.
2.1. Review of TCP MD5 Option
For review, the TCP MD5 option is shown in Figure 1.
+---------+---------+-------------------+
| Kind=19 |Length=18| MD5 digest... |
+---------+---------+-------------------+
| ...digest (con't)... |
+---------------------------------------+
| ... |
+---------------------------------------+
| ... |
+-------------------+-------------------+
| ...digest (con't) |
+-------------------+
Figure 1: The TCP MD5 Option [RFC2385]
In the TCP MD5 option, the length is fixed, and the MD5 digest
occupies 16 bytes following the Kind and Length fields (each one
byte), using the full MD5 digest of 128 bits [RFC1321].
The TCP MD5 option specifies the use of the MD5 digest calculation
over the following values in the following order:
1. The IP pseudoheader (IP source and destination addresses, protocol
number, and segment length).
Touch, et al. Standards Track [Page 7]

RFC 5925 The TCP Authentication Option June 2010
2. The TCP header excluding options and checksum.
3. The TCP data payload.
4. A key.
2.2. The TCP Authentication Option Format
TCP-AO provides a superset of the capabilities of TCP MD5, and is
minimal in the spirit of SP4 [SDNS88]. TCP-AO uses a new Kind field,
and similar Length field to TCP MD5, a KeyID field, and a RNextKeyID
field as shown in Figure 2.
+------------+------------+------------+------------+
| Kind=29 | Length | KeyID | RNextKeyID |
+------------+------------+------------+------------+
| MAC ...
+-----------------------------------...
...-----------------+
... MAC (con't) |
...-----------------+
Figure 2: The TCP Authentication Option (TCP-AO)
TCP-AO defines these fields as follows:
o Kind: An unsigned 1-byte field indicating TCP-AO. TCP-AO uses a
new Kind value of 29.
>> An endpoint MUST NOT use TCP-AO for the same connection in
which TCP MD5 is used. When both options appear, TCP MUST
silently discard the segment.
>> A single TCP segment MUST NOT have more than one TCP-AO in its
options sequence. When multiple TCP-AOs appear, TCP MUST discard
the segment.
o Length: An unsigned 1-byte field indicating the length of the
option in bytes, including the Kind, Length, KeyID, RNextKeyID,
and MAC fields.
>> The Length value MUST be greater than or equal to 4. When the
Length value is less than 4, TCP MUST discard the segment.
>> The Length value MUST be consistent with the TCP header length.
When the Length value is invalid, TCP MUST discard the segment.
Touch, et al. Standards Track [Page 8]

RFC 5925 The TCP Authentication Option June 2010
This Length check implies that the sum of the sizes of all
options, when added to the size of the base TCP header (5 words),
matches the TCP Offset field exactly. This full verification can
be computed because RFC 793 specifies the size of the required
options, and RFC 1122 requires that all new options follow a
common format with a fixed-length field location
[RFC793][RFC1122]. A partial verification can be limited to check
only TCP-AO, so that the TCP-AO length, when added to the TCP-AO
offset from the start of the TCP header, does not exceed the TCP
header size as indicated in the TCP header Offset field.
Values of 4 and other small values larger than 4 (e.g., indicating
MAC fields of very short length) are of dubious utility but are
not specifically prohibited.
o KeyID: An unsigned 1-byte field indicating the Master Key Tuple
(MKT, as defined in Section 3.1) used to generate the traffic keys
that were used to generate the MAC that authenticates this
segment.
It supports efficient key changes during a connection and/or to
help with key coordination during connection establishment, to be
discussed further in Section 6.1. Note that the KeyID has no
cryptographic properties -- it need not be random, nor are there
any reserved values.
>> KeyID values MAY be the same in both directions of a
connection, but do not have to be and there is no special meaning
when they are.
This allows MKTs to be installed on a set of devices without
coordinating the KeyIDs across that entire set in advance, and
allows new devices to be added to that set using a group of MKTs
later without requiring renumbering of KeyIDs. These two
capabilities are particularly important when used with wildcards
in the TCP socket pair of the MKT, i.e., when an MKT is used among
a set of devices specified by a pattern (as noted in Section 3.1).
o RNextKeyID: An unsigned 1-byte field indicating the MKT that is
ready at the sender to be used to authenticate received segments,
i.e., the desired 'receive next' key ID.
It supports efficient key change coordination, to be discussed
further in Section 6.1. Note that the RNextKeyID has no
cryptographic properties -- it need not be random, nor are there
any reserved values.
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RFC 5925 The TCP Authentication Option June 2010
o MAC: Message Authentication Code. Its contents are determined by
the particulars of the security association. Typical MACs are
96-128 bits (12-16 bytes), but any length that fits in the header
of the segment being authenticated is allowed. The MAC
computation is described further in Section 5.1.
>> Required support for TCP-AO MACs is defined in [RFC5926]; other
MACs MAY be supported.
TCP-AO fields do not indicate the MAC algorithm either implicitly (as
with TCP MD5) or explicitly. The particular algorithm used is
considered part of the configuration state of the connection's
security and is managed separately (see Section 3).
Please note that the use of TCP-AO does not affect TCP's advertised
Maximum Segment Size (MSS), as is the case for all TCP options
[Bo09].
The remainder of this document explains how TCP-AO is handled and its
relationship to TCP.
3. TCP-AO Keys and Their Properties
TCP-AO relies on two sets of keys to authenticate incoming and
outgoing segments: Master Key Tuples (MKTs) and traffic keys. MKTs
are used to derive unique traffic keys, and include the keying
material used to generate those traffic keys, as well as indicating
the associated parameters under which traffic keys are used. Such
parameters include whether TCP options are authenticated, and
indicators of the algorithms used for traffic key derivation and MAC
calculation. Traffic keys are the keying material used to compute
the MAC of individual TCP segments.
3.1. Master Key Tuple
A Master Key Tuple (MKT) describes TCP-AO properties to be associated
with one or more connections. It is composed of the following:
o TCP connection identifier. A TCP socket pair, i.e., a local IP
address, a remote IP address, a TCP local port, and a TCP remote
port. Values can be partially specified using ranges (e.g.,
2-30), masks (e.g., 0xF0), wildcards (e.g., "*"), or any other
suitable indication.
o TCP option flag. This flag indicates whether TCP options other
than TCP-AO are included in the MAC calculation. When options are
included, the content of all options, in the order present, is
included in the MAC, with TCP-AO's MAC field zeroed out. When the
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RFC 5925 The TCP Authentication Option June 2010
options are not included, all options other than TCP-AO are
excluded from all MAC calculations (skipped over, not zeroed).
Note that TCP-AO, with its MAC field zeroed out, is always
included in the MAC calculation, regardless of the setting of this
flag; this protects the indication of the MAC length as well as
the key ID fields (KeyID, RNextKeyID). The option flag applies to
TCP options in both directions (incoming and outgoing segments).
o IDs. The values used in the KeyID or RNextKeyID of TCP-AO; used
to differentiate MKTs in concurrent use (KeyID), as well as to
indicate when MKTs are ready for use in the opposite direction
(RNextKeyID).
Each MKT has two IDs - -- a SendID and a RecvID. The SendID is
inserted as the KeyID of the TCP-AO option of outgoing segments,
and the RecvID is matched against the TCP-AO KeyID of incoming
segments. These and other uses of these two IDs are described
further in Sections 7.4 and 7.5.
>> MKT IDs MUST support any value, 0-255 inclusive. There are no
reserved ID values.
ID values are assigned arbitrarily, i.e., the values are not
monotonically increasing, have no reserved values, and are
otherwise not meaningful. They can be assigned in sequence, or
based on any method mutually agreed by the connection endpoints
(e.g., using an external MKT management mechanism).
>> IDs MUST NOT be assumed to be randomly assigned.
o Master key. A byte sequence used for generating traffic keys,
this may be derived from a separate shared key by an external
protocol over a separate channel. This sequence is used in the
traffic key generation algorithm described in Section 5.2.
Implementations are advised to keep master key values in a
private, protected area of memory or other storage.
o Key Derivation Function (KDF). Indicates the key derivation
function and its parameters, as used to generate traffic keys from
master keys. It is explained further in Section 5.2 of this
document and specified in detail in [RFC5926].
o Message Authentication Code (MAC) algorithm. Indicates the MAC
algorithm and its parameters as used for this connection. It is
explained further in Section 5.1 of this document and specified in
detail in [RFC5926].
Touch, et al. Standards Track [Page 11]

RFC 5925 The TCP Authentication Option June 2010
>> Components of an MKT MUST NOT change during a connection.
MKT component values cannot change during a connection because TCP
state is coordinated during connection establishment. TCP lacks a
handshake for modifying that state after a connection has been
established.
>> The set of MKTs MAY change during a connection.
MKT parameters are not changed. Instead, new MKTs can be installed,
and a connection can change which MKT it uses.
>> The IDs of MKTs MUST NOT overlap where their TCP connection
identifiers overlap.
This document does not address how MKTs are created by users or
processes. It is presumed that an MKT affecting a particular
connection cannot be destroyed during an active connection -- or,
equivalently, that its parameters are copied to an area local to the
connection (i.e., instantiated) and so changes would affect only new
connections. The MKTs can be managed by a separate application
protocol.
3.2. Traffic Keys
A traffic key is a key derived from the MKT and the local and remote
IP address pairs and TCP port numbers, and, for established
connections, the TCP Initial Sequence Numbers (ISNs) in each
direction. Segments exchanged before a connection is established use
the same information, substituting zero for unknown values (e.g.,
ISNs not yet coordinated).
A single MKT can be used to derive any of four different traffic
keys:
o Send_SYN_traffic_key
o Receive_SYN_traffic_key
o Send_other_traffic_key
o Receive_other_traffic_key
Note that the keys are unidirectional. A given connection typically
uses only three of these keys, because only one of the SYN keys is
typically used. All four are used only when a connection goes
through 'simultaneous open' [RFC793].
Touch, et al. Standards Track [Page 12]

RFC 5925 The TCP Authentication Option June 2010
We recommend that the mechanism used to select from among multiple
MKTs use only information that TCP-AO would authenticate. Because
MKTs may indicate that options other than TCP-AO are ignored in the
MAC calculation, we recommend that TCP options should not be used to
determine MKTs.
>> An incoming TCP segment including TCP-AO MUST match exactly one
MKT, indicated solely by the segment's socket pair and its TCP-AO
KeyID.
Incoming segments include an indicator inside TCP-AO to select from
among multiple matching MKTs -- the KeyID field. TCP-AO requires
that the KeyID alone be used to differentiate multiple matching MKTs,
so that MKT changes can be coordinated using the TCP-AO key change
coordination mechanism.
>> When an outgoing TCP segment matches no MKTs, TCP-AO is not used.
TCP-AO is always used when outgoing segments match an MKT, and is not
used otherwise.
4. Per-Connection TCP-AO Parameters
TCP-AO uses a small number of parameters associated with each
connection that uses TCP-AO, once instantiated. These values can be
stored in the Transport Control Block (TCB) [RFC793]. These values
are explained in subsequent sections of this document as noted; they
include:
1. Current_key - the MKT currently used to authenticate outgoing
segments, whose SendID is inserted in outgoing segments as KeyID
(see Section 7.4, step 2.f). Incoming segments are authenticated
using the MKT corresponding to the segment and its TCP-AO KeyID
(see Section 7.5, step 2.c), as matched against the MKT TCP
connection identifier and the MKT RecvID. There is only one
current_key at any given time on a particular connection.
>> Every TCP connection in a non-IDLE state MUST have at most one
current_key specified.
2. Rnext_key - the MKT currently preferred for incoming (received)
segments, whose RecvID is inserted in outgoing segments as
RNextKeyID (see Section 7.4, step 2.d).
>> Each TCP connection in a non-IDLE state MUST have at most one
rnext_key specified.
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RFC 5925 The TCP Authentication Option June 2010
3. A pair of Sequence Number Extensions (SNEs). SNEs are used to
prevent replay attacks, as described in Section 6.2. Each SNE is
initialized to zero upon connection establishment. Its use in the
MAC calculation is described in Section 5.1.
4. One or more MKTs. These are the MKTs that match this connection's
socket pair.
MKTs are used, together with other parameters of a connection, to
create traffic keys unique to each connection, as described in
Section 5.2. These traffic keys can be cached after computation, and
can be stored in the TCB with the corresponding MKT information.
They can be considered part of the per-connection parameters.
5. Cryptographic Algorithms
TCP-AO uses cryptographic algorithms to compute the MAC (Message
Authentication Code) that is used to authenticate a segment and its
headers; these are called MAC algorithms and are specified in a
separate document to facilitate updating the algorithm requirements
independently from the protocol [RFC5926]. TCP-AO also uses
cryptographic algorithms to convert MKTs, which can be shared across
connections, into unique traffic keys for each connection. These are
called Key Derivation Functions (KDFs) and are specified [RFC5926].
This section describes how these algorithms are used by TCP-AO.
5.1. MAC Algorithms
MAC algorithms take a variable-length input and a key and output a
fixed-length number. This number is used to determine whether the
input comes from a source with that same key, and whether the input
has been tampered with in transit. MACs for TCP-AO have the
following interface:
MAC = MAC_alg(traffic_key, message)
INPUT: MAC_alg, traffic_key, message
OUTPUT: MAC
where:
o MAC_alg - the specific MAC algorithm used for this computation.
The MAC algorithm specifies the output length, so no separate
output length parameter is required. This is specified as
described in [RFC5926].
Touch, et al. Standards Track [Page 15]

RFC 5925 The TCP Authentication Option June 2010
o Traffic_key - traffic key used for this computation. This is
computed from the connection's current MKT as described in Section5.2.
o Message - input data over which the MAC is computed. In TCP-AO,
this is the TCP segment prepended by the IP pseudoheader and TCP
header options, as described in Section 5.1.
o MAC - the fixed-length output of the MAC algorithm, given the
parameters provided.
At the time of this writing, the algorithms' definitions for use in
TCP-AO, as described in [RFC5926], are each truncated to 96 bits.
Though the algorithms each output a larger MAC, 96 bits provides a
reasonable trade-off between security and message size. However,
this could change in the future, so TCP-AO size should not be assumed
as fixed length.
The MAC algorithm employed for the MAC computation on a connection is
done so by definition in the MKT, per the definition in [RFC5926].
The mandatory-to-implement MAC algorithms for use with TCP-AO are
described in a separate RFC [RFC5926]. This allows the TCP-AO
specification to proceed along the IETF Standards Track even if
changes are needed to its associated algorithms and their labels (as
might be used in a user interface or automated MKT management
protocol) as a result of the ever evolving world of cryptography.
>> Additional algorithms, beyond those mandated for TCP-AO, MAY be
supported.
The data input to the MAC is in the following fields in the following
sequence, interpreted in network-standard byte order:
1. The Sequence Number Extension (SNE), in network-standard byte
order, as follows (described further in Section 6.2):
+--------+--------+--------+--------+
| SNE |
+--------+--------+--------+--------+
Figure 4: Sequence Number Extension
The SNE for transmitted segments is maintained locally in the
SND.SNE value; for received segments, a local RCV.SNE value is
used. The details of how these values are maintained and used are
in Sections 6.2, 7.4, and 7.5.
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RFC 5925 The TCP Authentication Option June 2010
When TCP options are not included, all TCP options except for TCP-
AO are omitted from MAC processing. Again, the TCP-AO MAC field
is zeroed for the MAC processing.
4. The TCP data, i.e., the payload of the TCP segment.
Note that the traffic key is not included as part of the data; the
MAC algorithm indicates how to use the traffic key, for example,
as HMACs do [RFC2104][RFC2403]. The traffic key is derived from
the current MKT as described in Section 5.2.
5.2. Traffic Key Derivation Functions
TCP-AO's traffic keys are derived from the MKTs using Key Derivation
Functions (KDFs). The KDFs used in TCP-AO have the following
interface:
traffic_key = KDF_alg(master_key, context, output_length)
INPUT: KDF_alg, master_key, context, output_length
OUTPUT: traffic_key
where:
o KDF_alg - The specific Key Derivation Function (KDF) that is the
basic building block used in constructing the traffic key, as
indicated in the MKT. This is specified as described in
[RFC5926].
o Master_key - The master_key string, as will be stored into the
associated MKT.
o Context - The context used as input in constructing the
traffic_key, as specified in [RFC5926]. The specific way this
context is used, in conjunction with other information, to create
the raw input to the KDF is also explained further in [RFC5926].
o Output_length - The desired output length of the KDF, i.e., the
length to which the KDF's output will be truncated. This is
specified as described in [RFC5926].
o Traffic_key - The desired output of the KDF, of length
output_length, to be used as input to the MAC algorithm, as
described in Section 5.1.
Touch, et al. Standards Track [Page 18]

RFC 5925 The TCP Authentication Option June 2010
+--------+--------+--------+--------+
| |
+ +
| |
+ Source Address +
| |
+ +
| |
+ +
+--------+--------+--------+--------+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+--------+--------+--------+--------+
| Source Port | Dest. Port |
+--------+--------+--------+--------+
| Source ISN |
+--------+--------+--------+--------+
| Dest. ISN |
+--------+--------+--------+--------+
Figure 8: KDF Context for an IPv6 Connection
Traffic keys are directional, so "source" and "destination" are
interpreted differently for incoming and outgoing segments. For
incoming segments, source is the remote side; whereas for outgoing
segments, source is the local side. This further ensures that
connection keys generated for each direction are unique.
For SYN segments (segments with the SYN set, but the ACK not set),
the destination ISN is not known. For these segments, the connection
key is computed using the context shown above, in which the
destination ISN value is zero. For all other segments, the ISN pair
is used when known. If the ISN pair is not known, e.g., when sending
a reset (RST) after a reboot, the segment should be sent without
authentication; if authentication was required, the segment cannot
have been MAC'd properly anyway and would have been dropped on
receipt.
>> TCP-AO SYN segments (SYN set, no ACK set) MUST use a destination
ISN of zero (whether sent or received); all other segments use the
known ISN pair.
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RFC 5925 The TCP Authentication Option June 2010
Overall, this means that each connection will use up to four distinct
traffic keys for each MKT:
o Send_SYN_traffic_key - the traffic key used to authenticate
outgoing SYNs. The source ISN is known (the TCP connection's
local ISN), and the destination (remote) ISN is unknown (and so
the value 0 is used).
o Receive_SYN_traffic_key - the traffic key used to authenticate
incoming SYNs. The source ISN is known (the TCP connection's
remote ISN), and the destination (remote) ISN is unknown (and so
the value 0 is used).
o Send_other_traffic_key - the traffic key used to authenticate all
other outgoing TCP segments.
o Receive_other_traffic_key - the traffic key used to authenticate
all other incoming TCP segments.
The following table describes how each of these traffic keys is
computed, where the TCP-AO algorithms refer to source (S) and
destination (D) values of the IP address, TCP port, and ISN, and each
segment (incoming or outgoing) has a value that refers to the local
side of the connection (l) and remote side (r):
S-IP S-port S-ISN D-IP D-port D-ISN
----------------------------------------------------------------
Send_SYN_traffic_key l-IP l-port l-ISN r-IP r-port 0
Receive_SYN_traffic_key r-IP r-port r-ISN l-IP l-port 0
Send_other_traffic_key l-IP l-port l-ISN r-IP r-port r-ISN
Receive_other_traffic_key r-IP r-port r-ISN l-IP l-port l-ISN
The use of both ISNs in the traffic key computations ensures that
segments cannot be replayed across repeated connections reusing the
same socket; their 32-bit space avoids repeated use except under
reboot, and reuse assumes both sides repeat their use on the same
connection. We do expect that:
>> Endpoints should select ISNs pseudorandomly, e.g., as in
[RFC1948].
A SYN is authenticated using a destination ISN of zero (whether sent
or received), and all other segments would be authenticated using the
ISN pair for the connection. There are other cases in which the
destination ISN is not known, but segments are emitted, such as after
an endpoint reboots, when it is possible that the two endpoints would
not have enough information to authenticate segments. This is
addressed further in Section 7.7.
Touch, et al. Standards Track [Page 21]

RFC 5925 The TCP Authentication Option June 20105.3. Traffic Key Establishment and Duration Issues
TCP-AO does not provide a mechanism for traffic key negotiation or
parameter negotiation (MAC algorithm, length, or use of TCP-AO on a
connection), or for coordinating rekeying during a connection. We
assume out-of-band mechanisms for MKT establishment, parameter
negotiation, and rekeying. This separation of MKT use from MKT
management is similar to that in the IPsec suite [RFC4301][RFC4306].
We encourage users of TCP-AO to apply known techniques for generating
appropriate MKTs, including the use of reasonable master key lengths,
limited traffic key sharing, and limiting the duration of MKT use
[RFC3562]. This also includes the use of per-connection nonces, as
suggested in Section 5.2.
TCP-AO supports rekeying in which new MKTs are negotiated and
coordinated out of band, either via a protocol or a manual procedure
[RFC4808]. New MKT use is coordinated using the out-of-band
mechanism to update both TCP endpoints. When only a single MKT is
used at a time, the temporary use of invalid MKTs could result in
segments being dropped; although TCP is already robust to such drops,
TCP-AO uses the KeyID field to avoid such drops. A given connection
can have multiple matching MKTs, where the KeyID field is used to
identify the MKT that corresponds to the traffic key used for a
segment, to avoid the need for expensive trial-and-error testing of
MKTs in sequence.
TCP-AO provides an explicit MKT coordination mechanism, described in
Section 6.1. Such a mechanism is useful when new MKTs are installed,
or when MKTs are changed, to determine when to commence using
installed MKTs.
Users are advised to manage MKTs following the spirit of the advice
for key management when using TCP MD5 [RFC3562], notably to use
appropriate key lengths (12-24 bytes) and to avoid sharing MKTs among
multiple BGP peering arrangements.
5.3.1. MKT Reuse Across Socket Pairs
MKTs can be reused across different socket pairs within a host, or
across different instances of a socket pair within a host. In either
case, replay protection is maintained.
MKTs reused across different socket pairs cannot enable replay
attacks because the TCP socket pair is included in the MAC, as well
as in the generation of the traffic key. MKTs reused across repeated
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RFC 5925 The TCP Authentication Option June 2010
instances of a given socket pair cannot enable replay attacks because
the connection ISNs are included in the traffic key generation
algorithm, and ISN pairs are unlikely to repeat over useful periods.
5.3.2. MKTs Use within a Long-Lived Connection
TCP-AO uses Sequence Number Extensions (SNEs) to prevent replay
attacks within long-lived connections. Explicit MKT rollover,
accomplished by external means and indexed using the KeyID field, can
be used to change keying material for various reasons (e.g.,
personnel turnover), but is not required to support long-lived
connections.
6. Additional Security Mechanisms
TCP-AO adds mechanisms to support efficient use, especially in
environments where only manual keying is available. These include
the previously described mechanisms for supporting multiple
concurrent MKTs (via the KeyID field) and for generating unique per-
connection traffic keys (via the KDF). This section describes
additional mechanisms to coordinate MKT changes and to prevent replay
attacks when a traffic key is not changed for long periods of time.
6.1. Coordinating Use of New MKTs
At any given time, a single TCP connection may have multiple MKTs
specified for each segment direction (incoming, outgoing). TCP-AO
provides a mechanism to indicate when a new MKT is ready, which
allows the sender to commence use of that new MKT. This mechanism
allows new MKT use to be coordinated, to avoid unnecessary loss due
to sender authentication using an MKT not yet ready at the receiver.
Note that this is intended as an optimization. Deciding when to
start using a key is a performance issue. Deciding when to remove an
MKT is a security issue. Invalid MKTs are expected to be removed.
TCP-AO provides no mechanism to coordinate their removal, as we
consider this a key management operation.
New MKT use is coordinated through two ID fields in the header:
o KeyID
o RNextKeyID
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RFC 5925 The TCP Authentication Option June 2010
KeyID represents the outgoing MKT information used by the segment
sender to create the segment's MAC (outgoing), and the corresponding
incoming keying information used by the segment receiver to validate
that MAC. It contains the SendID of the MKT in active use in that
direction.
RNextKeyID represents the preferred MKT information to be used for
subsequent received segments ('receive next'). That is, it is a way
for the segment sender to indicate a ready incoming MKT for future
segments it receives, so that the segment receiver can know when to
switch MKTs (and thus their KeyIDs and associated traffic keys). It
indicates the RecvID of the MKT desired for incoming segments.
There are two pointers kept by each side of a connection, as noted in
the per-connection information (see Section 4):
o Currently active outgoing MKT (current_key)
o Current preference for incoming MKT (rnext_key)
Current_key indicates an MKT that is used to authenticate outgoing
segments. Upon connection establishment, it points to the first MKT
selected for use.
Rnext_key points to an incoming MKT that is ready and preferred for
use. Upon connection establishment, this points to the currently
active incoming MKT. It can be changed when new MKTs are installed
(e.g., by either automatic MKT management protocol operation or user
manual selection).
Rnext_key is changed only by manual user intervention or MKT
management protocol operation. It is not manipulated by TCP-AO.
Current_key is updated by TCP-AO when processing received TCP
segments as discussed in the segment processing description in
Section 7.5. Note that the algorithm allows the current_key to
change to a new MKT, then change back to a previously used MKT (known
as "backing up"). This can occur during an MKT change when segments
are received out of order, and is considered a feature of TCP-AO,
because reordering does not result in drops. The only way to avoid
reuse of previously used MKTs is to remove the MKT when it is no
longer considered permitted.
6.2. Preventing Replay Attacks within Long-Lived Connections
TCP uses a 32-bit sequence number, which may, for long-lived
connections, roll over and repeat. This could result in TCP segments
being intentionally and legitimately replayed within a connection.
TCP-AO prevents replay attacks, and thus requires a way to
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RFC 5925 The TCP Authentication Option June 2010
differentiate these legitimate replays from each other, and so it
adds a 32-bit Sequence Number Extension (SNE) for transmitted and
received segments.
The SNE extends the TCP sequence number so that segments within a
single connection are always unique. When the TCP's sequence number
rolls over, there is a chance that a segment could be repeated in
total; using an SNE differentiates even identical segments sent with
identical sequence numbers at different times in a connection. TCP-
AO emulates a 64-bit sequence number space by inferring when to
increment the high-order 32-bit portion (the SNE) based on
transitions in the low-order portion (the TCP sequence number).
TCP-AO thus maintains SND.SNE for transmitted segments, and RCV.SNE
for received segments, both initialized as zero when a connection
begins. The intent of these SNEs is, together with TCP's 32-bit
sequence numbers, to provide a 64-bit overall sequence number space.
For transmitted segments, SND.SNE can be implemented by extending
TCP's sequence number to 64 bits; SND.SNE would be the top (high-
order) 32 bits of that number. For received segments, TCP-AO needs
to emulate the use of a 64-bit number space and correctly infer the
appropriate high-order 32-bits of that number as RCV.SNE from the
received 32-bit sequence number and the current connection context.
The implementation of SNEs is not specified in this document, but one
possible way is described here that can be used for either RCV.SNE,
SND.SNE, or both.
Consider an implementation with two SNEs as required (SND.SNE, RCV.
SNE), and additional variables as listed below, all initialized to
zero, as well as a current TCP segment field (SEG.SEQ):
o SND.PREV_SEQ, needed to detect rollover of SND.SEQ
o RCV.PREV_SEQ, needed to detect rollover of RCV.SEQ
o SND.SNE_FLAG, which indicates when to increment the SND.SNE
o RCV.SNE_FLAG, which indicates when to increment the RCV.SNE
When a segment is received, the following algorithm (in C-like
pseudocode) computes the SNE used in the MAC; this is the "RCV" side,
and an equivalent algorithm can be applied to the "SND" side:
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RFC 5925 The TCP Authentication Option June 2010
/* set the flag when the SEG.SEQ first rolls over */
if ((RCV.SNE_FLAG == 0)
&& (RCV.PREV_SEQ > 0x7fff) && (SEG.SEQ < 0x7fff)) {
RCV.SNE = RCV.SNE + 1;
RCV.SNE_FLAG = 1;
}
/* decide which SNE to use after incremented */
if ((RCV.SNE_FLAG == 1) && (SEG.SEQ > 0x7fff)) {
SNE = RCV.SNE - 1; # use the pre-increment value
} else {
SNE = RCV.SNE; # use the current value
}
/* reset the flag in the *middle* of the window */
if ((RCV.PREV_SEQ < 0x7fff) && (SEG.SEQ > 0x7fff)) {
RCV.SNE_FLAG = 0;
}
/* save the current SEQ for the next time through the code */
RCV.PREV_SEQ = SEG.SEQ;
In the above code, the first time the sequence number rolls over,
i.e., when the new number is low (in the bottom half of the number
space) and the old number is high (in the top half of the number
space), the SNE is incremented and a flag is set.
If the flag is set and a high number is seen, it must be a reordered
segment, so use the pre-increment SNE; otherwise, use the current
SNE.
The flag will be cleared by the time the number rolls all the way
around.
The flag prevents the SNE from being incremented again until the flag
is reset, which happens in the middle of the window (when the old
number is in the bottom half and the new is in the top half).
Because the receive window is never larger than half of the number
space, it is impossible to both set and reset the flag at the same
time -- outstanding segments, regardless of reordering, cannot
straddle both regions simultaneously.
7. TCP-AO Interaction with TCP
The following is a description of how TCP-AO affects various TCP
states, segments, events, and interfaces. This description is
intended to augment the description of TCP as provided in RFC 793,
and its presentation mirrors that of RFC 793 as a result [RFC793].
Touch, et al. Standards Track [Page 26]

RFC 5925 The TCP Authentication Option June 20107.1. TCP User Interface
The TCP user interface supports active and passive OPEN, SEND,
RECEIVE, CLOSE, STATUS, and ABORT commands. TCP-AO does not alter
this interface as it applies to TCP, but some commands or command
sequences of the interface need to be modified to support TCP-AO.
TCP-AO does not specify the details of how this is achieved.
TCP-AO requires that the TCP user interface be extended to allow the
MKTs to be configured, as well as to allow an ongoing connection to
manage which MKTs are active. The MKTs need to be configured prior
to connection establishment, and the set of MKTs may change during a
connection:
>> TCP OPEN, or the sequence of commands that configure a connection
to be in the active or passive OPEN state, MUST be augmented so that
an MKT can be configured.
>> A TCP-AO implementation MUST allow the set of MKTs for ongoing TCP
connections (i.e., not in the CLOSED state) to be modified.
The MKTs associated with a connection need to be available for
confirmation; this includes the ability to read the MKTs:
>> TCP STATUS SHOULD be augmented to allow the MKTs of a current or
pending connection to be read (for confirmation).
Senders may need to be able to determine when the outgoing MKT
changes (KeyID) or when a new preferred MKT (RNextKeyID) is
indicated; these changes immediately affect all subsequent outgoing
segments:
>> TCP SEND, or a sequence of commands resulting in a SEND, MUST be
augmented so that the preferred outgoing MKT (current_key) and/or the
preferred incoming MKT (rnext_key) of a connection can be indicated.
It may be useful to change the outgoing active MKT (current_key) even
when no data is being sent, which can be achieved by sending a zero-
length buffer or by using a non-send interface (e.g., socket options
in Unix), depending on the implementation.
It is also useful to indicate recent segment KeyID and RNextKeyID
values received; although there could be a number of such values,
they are not expected to change quickly, so any recent sample should
be sufficient:
Touch, et al. Standards Track [Page 27]

RFC 5925 The TCP Authentication Option June 2010
>> TCP RECEIVE, or the sequence of commands resulting in a RECEIVE,
MUST be augmented so that the KeyID and RNextKeyID of a recently
received segment is available to the user out of band (e.g., as an
additional parameter to RECEIVE or via a STATUS call).
7.2. TCP States and Transitions
TCP includes the states LISTEN, SYN-SENT, SYN-RECEIVED, ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and
CLOSED.
>> An MKT MAY be associated with any TCP state.
7.3. TCP Segments
TCP includes control (at least one of SYN, FIN, RST flags set) and
data (none of SYN, FIN, or RST flags set) segments. Note that some
control segments can include data (e.g., SYN).
>> All TCP segments MUST be checked against the set of MKTs for
matching TCP connection identifiers.
>> TCP segments whose TCP-AO does not validate MUST be silently
discarded.
>> A TCP-AO implementation MUST allow for configuration of the
behavior of segments with TCP-AO but that do not match an MKT. The
initial default of this configuration SHOULD be to silently accept
such connections. If this is not the desired case, an MKT can be
included to match such connections, or the connection can indicate
that TCP-AO is required. Alternately, the configuration can be
changed to discard segments with the AO option not matching an MKT.
>> Silent discard events SHOULD be signaled to the user as a warning,
and silent accept events MAY be signaled to the user as a warning.
Both warnings, if available, MUST be accessible via the STATUS
interface. Either signal MAY be asynchronous, but if so, they MUST
be rate-limited. Either signal MAY be logged; logging SHOULD allow
rate-limiting as well.
All TCP-AO processing occurs between the interface of TCP and IP; for
incoming segments, this occurs after validation of the TCP checksum.
For outgoing segments, this occurs before computation of the TCP
checksum.
Touch, et al. Standards Track [Page 28]

RFC 5925 The TCP Authentication Option June 2010
Note that use of TCP-AO on a connection is not negotiated within TCP.
It is the responsibility of the receiver to determine when TCP-AO is
required via other means (e.g., out of band, manually or with a key
management protocol) and to enforce that requirement.
7.4. Sending TCP Segments
The following procedure describes the modifications to TCP to support
inserting TCP-AO when a segment departs.
>> Note that TCP-AO MUST be the last TCP option processed on outgoing
segments, because its MAC calculation may include the values of other
TCP options.
1. Find the per-connection parameters for the segment:
a. If the segment is a SYN, then this is the first segment of a
new connection. Find the matching MKT for this segment based
on the segment's socket pair.
i. If there is no matching MKT, omit TCP-AO. Proceed with
transmitting the segment.
ii. If there is a matching MKT, then set the per-connection
parameters as needed (see Section 4). Proceed with the
step 2.
b. If the segment is not a SYN, then determine whether TCP-AO is
being used for the connection and use the MKT as indicated by
the current_key value from the per-connection parameters (see
Section 4) and proceed with the step 2.
2. Using the per-connection parameters:
a. Augment the TCP header with TCP-AO, inserting the appropriate
Length and KeyID based on the MKT indicated by current_key
(using the current_key MKT's SendID as the TCP-AO KeyID).
Update the TCP header length accordingly.
b. Determine SND.SNE as described in Section 6.2.
c. Determine the appropriate traffic key, i.e., as pointed to by
the current_key (as noted in Section 6.1, and as probably
cached in the TCB). That is, use the send_SYN_traffic_key for
SYN segments and the send_other_traffic_key for other
segments.
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RFC 5925 The TCP Authentication Option June 2010
d. Determine the RNextKeyID as indicated by the rnext_key
pointer, and insert it in the TCP-AO RNextKeyID field (using
the rnext_key MKT's RecvID as the TCP-AO KeyID) (as noted in
Section 6.1).
e. Compute the MAC using the MKT (and cached traffic key) and
data from the segment as specified in Section 5.1.
f. Insert the MAC in the TCP-AO MAC field.
g. Proceed with transmitting the segment.
7.5. Receiving TCP Segments
The following procedure describes the modifications to TCP to support
TCP-AO when a segment arrives.
>> Note that TCP-AO MUST be the first TCP option processed on
incoming segments, because its MAC calculation may include the values
of other TCP options that could change during TCP option processing.
This also protects the behavior of all other TCP options from the
impact of spoofed segments or modified header information.
>> Note that TCP-AO checks MUST be performed for all incoming SYNs to
avoid accepting SYNs lacking TCP-AO where required. Other segments
can cache whether TCP-AO is needed in the TCB.
1. Find the per-connection parameters for the segment:
a. If the segment is a SYN, then this is the first segment of a
new connection. Find the matching MKT for this segment, using
the segment's socket pair and its TCP-AO KeyID, matched
against the MKT's TCP connection identifier and the MKT's
RecvID.
i. If there is no matching MKT, remove TCP-AO from the
segment. Proceed with further TCP handling of the segment.
NOTE: this presumes that connections that do not match any
MKT should be silently accepted, as noted in Section 7.3.
ii. If there is a matching MKT, then set the per-connection
parameters as needed (see Section 4). Proceed with step 2.
Touch, et al. Standards Track [Page 30]

RFC 5925 The TCP Authentication Option June 2010
2. Using the per-connection parameters:
a. Check that the segment's TCP-AO Length matches the length
indicated by the MKT.
i. If the lengths differ, silently discard the segment. Log
and/or signal the event as indicated in Section 7.3.
b. Determine the segment's RCV.SNE as described in Section 6.2.
c. Determine the segment's traffic key from the MKT as described
in Section 5.1 (and as likely cached in the TCB). That is,
use the receive_SYN_traffic_key for SYN segments and the
receive_other_traffic_key for other segments.
d. Compute the segment's MAC using the MKT (and its derived
traffic key) and portions of the segment as indicated in
Section 5.1.
i. If the computed MAC differs from the TCP-AO MAC field
value, silently discard the segment. Log and/or signal the
event as indicated in Section 7.3.
e. Compare the received RNextKeyID value to the currently active
outgoing KeyID value (current_key MKT's SendID).
i. If they match, no further action is required.
ii. If they differ, determine whether the RNextKeyID MKT is
ready.
1. If the MKT corresponding to the segment's socket pair
and RNextKeyID is not available, no action is required
(RNextKeyID of a received segment needs to match the
MKT's SendID).
2. If the matching MKT corresponding to the segment's
socket pair and RNextKeyID is available:
a. Set current_key to the RNextKeyID MKT.
f. Proceed with TCP processing of the segment.
It is suggested that TCP-AO implementations validate a segment's
Length field before computing a MAC to reduce the overhead incurred
by spoofed segments with invalid TCP-AO fields.
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RFC 5925 The TCP Authentication Option June 2010
Additional reductions in MAC validation overhead can be supported in
the MAC algorithms, e.g., by using a computation algorithm that
prepends a fixed value to the computed portion and a corresponding
validation algorithm that verifies the fixed value before investing
in the computed portion. Such optimizations would be contained in
the MAC algorithm specification, and thus are not specified in TCP-AO
explicitly. Note that the KeyID cannot be used for connection
validation per se, because it is not assumed random.
7.6. Impact on TCP Header Size
TCP-AO, using the initially required 96-bit MACs, uses a total of 16
bytes of TCP header space [RFC5926]. TCP-AO is thus 2 bytes smaller
than the TCP MD5 option (18 bytes).
Note that the TCP option space is most critical in SYN segments,
because flags in those segments could potentially increase the option
space area in other segments. Because TCP ignores unknown segments,
however, it is not possible to extend the option space of SYNs
without breaking backward compatibility.
TCP's 4-bit data offset requires that the options end 60 bytes (15
32-bit words) after the header begins, including the 20-byte header.
This leaves 40 bytes for options, of which 15 are expected in current
implementations (listed below), leaving at most 25 for other uses.
TCP-AO consumes 16 bytes, leaving 9 bytes for additional SYN options
(depending on implementation dependant alignment padding, which could
consume another 2 bytes at most).
o SACK permitted (2 bytes) [RFC2018][RFC3517]
o Timestamps (10 bytes) [RFC1323]
o Window scale (3 bytes) [RFC1323]
After a SYN, the following options are expected in current
implementations of TCP:
o SACK (10bytes) [RFC2018][RFC3517] (18 bytes if D-SACK [RFC2883])
o Timestamps (10 bytes) [RFC1323]
TCP-AO continues to consume 16 bytes in non-SYN segments, leaving a
total of 24 bytes for other options, of which the timestamp consumes
10. This leaves 14 bytes, of which 10 are used for a single SACK
block. When two SACK blocks are used, such as to handle D-SACK, a
smaller TCP-AO MAC would be required to make room for the additional
SACK block (i.e., to leave 18 bytes for the D-SACK variant of the
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RFC 5925 The TCP Authentication Option June 2010
SACK option) [RFC2883]. Note that D-SACK is not supportable in TCP
MD5 in the presence of timestamps, because TCP MD5's MAC length is
fixed and too large to leave sufficient option space.
Although TCP option space is limited, we believe TCP-AO is consistent
with the desire to authenticate TCP at the connection level for
similar uses as were intended by TCP MD5.
7.7. Connectionless Resets
TCP-AO allows TCP resets (RSTs) to be exchanged provided both sides
have established valid connection state. After such state is
established, if one side reboots, TCP-AO prevents TCP's RST mechanism
from clearing out old state on the side that did not reboot. This
happens because the rebooting side has lost its connection state, and
thus its traffic keys.
It is important that implementations are capable of detecting
excesses of TCP connections in such a configuration and can clear
them out if needed to protect its memory usage [Ba10]. To protect
against such state from accumulating and not being cleared out, a
number of recommendations are made:
>> Connections using TCP-AO SHOULD also use TCP keepalives [RFC1122].
The use of TCP keepalives ensures that connections whose keys are
lost are terminated after a finite time; a similar effect can be
achieved at the application layer, e.g., with BGP keepalives
[RFC4271]. Either kind of keepalive helps ensure the TCP state is
cleared out in such a case; the alternative, of allowing
unauthenticated RSTs to be received, would allow one of the primary
vulnerabilities that TCP-AO is intended to prevent.
Keepalives ensure that connections are dropped across reboots, but
this can have a detrimental effect on some protocols. Specifically,
BGP reacts poorly to such connection drops, even if caused by the use
of BGP keepalives; "graceful restart" was introduced to address this
effect [RFC4724], and extended to support BGP with MPLS [RFC4781].
As a result:
>> BGP connections SHOULD require support for graceful restart when
using TCP-AO.
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RFC 5925 The TCP Authentication Option June 2010
We recognize that support for graceful restart is not always
feasible. As a result:
>> When BGP without graceful restart is used with TCP-AO, both sides
of the connection SHOULD save traffic keys in storage that persists
across reboots and restore them after a reboot, and SHOULD limit any
performance impacts that result from this storage/restoration.
7.8. ICMP Handling
TCP can be attacked both in band, using TCP segments, or out of band
using ICMP. ICMP packets cannot be protected using TCP-AO
mechanisms; however, in this way, both TCP-AO and IPsec do not
directly solve the need for protected ICMP signaling. TCP-AO does
make specific recommendations on how to handle certain ICMPs, beyond
what IPsec requires, and these are made possible because TCP-AO
operates inside the context of a TCP connection.
IPsec makes recommendations regarding dropping ICMPs in certain
contexts or requiring that they are endpoint authenticated in others
[RFC4301]. There are other mechanisms proposed to reduce the impact
of ICMP attacks by further validating ICMP contents and changing the
effect of some messages based on TCP state, but these do not provide
the level of authentication for ICMP that TCP-AO provides for TCP
[Go10]. As a result, we recommend a conservative approach to
accepting ICMP messages as summarized in [Go10]:
>> A TCP-AO implementation MUST default to ignore incoming ICMPv4
messages of Type 3 (destination unreachable), Codes 2-4 (protocol
unreachable, port unreachable, and fragmentation needed -- 'hard
errors'), and ICMPv6 Type 1 (destination unreachable), Code 1
(administratively prohibited) and Code 4 (port unreachable) intended
for connections in synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-
WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT) that match MKTs.
>> A TCP-AO implementation SHOULD allow whether such ICMPs are
ignored to be configured on a per-connection basis.
>> A TCP-AO implementation SHOULD implement measures to protect ICMP
"packet too big" messages, some examples of which are discussed in
[Go10].
>> An implementation SHOULD allow ignored ICMPs to be logged.
This control affects only ICMPs that currently require 'hard errors',
which would abort the TCP connection [RFC1122]. This recommendation
is intended to be similar to how IPsec would handle those messages,
with an additional default assumed [RFC4301].
Touch, et al. Standards Track [Page 34]

RFC 5925 The TCP Authentication Option June 20108. Obsoleting TCP MD5 and Legacy Interactions
TCP-AO obsoletes TCP MD5. As we have noted earlier:
>> TCP implementations that support TCP MD5 MUST support TCP-AO.
Systems implementing TCP MD5 only are considered legacy, and ought to
be upgraded when possible. In order to support interoperation with
such legacy systems until upgrades are available:
>> TCP MD5 SHOULD be supported where interactions with legacy systems
are needed.
>> A system that supports both TCP-AO and TCP MD5 MUST use TCP-AO for
connections unless not supported by its peer, at which point it MAY
use TCP MD5 instead.
>> A TCP implementation MUST NOT use both TCP-AO and TCP MD5 for a
particular TCP connection, but MAY support TCP-AO and TCP MD5
simultaneously for different connections (notably to support legacy
use of TCP MD5).
The Kind value explicitly indicates whether TCP-AO or TCP MD5 is used
for a particular connection in TCP segments.
It is possible that MKTs could be augmented to support TCP MD5,
although use of MKTs is not described in RFC 2385.
It is possible to require TCP-AO for a connection or TCP MD5, but it
is not possible to require 'either'. When an endpoint is configured
to require TCP MD5 for a connection, it must be added to all outgoing
segments and validated on all incoming segments [RFC2385]. TCP MD5's
requirements prohibit the speculative use of both options for a given
connection, e.g., to be decided by the other end of the connection.
9. Interactions with Middleboxes
TCP-AO may interact with middleboxes, depending on their behavior
[RFC3234]. Some middleboxes either alter TCP options (such as TCP-
AO) directly or alter the information TCP-AO includes in its MAC
calculation. TCP-AO may interfere with these devices, exactly where
the device modifies information TCP-AO is designed to protect.
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RFC 5925 The TCP Authentication Option June 20109.1. Interactions with Non-NAT/NAPT Middleboxes
TCP-AO supports middleboxes that do not change the IP addresses or
ports of segments. Such middleboxes may modify some TCP options, in
which case TCP-AO would need to be configured to ignore all options
in the MAC calculation on connections traversing that element.
Note that ignoring TCP options may provide less protection, i.e., TCP
options could be modified in transit, and such modifications could be
used by an attacker. Depending on the modifications, TCP could have
compromised efficiency (e.g., timestamp changes), or could cease
correct operation (e.g., window scale changes). These
vulnerabilities affect only the TCP connections for which TCP-AO is
configured to ignore TCP options.
9.2. Interactions with NAT/NAPT Devices
TCP-AO cannot interoperate natively across NAT/NAPT (Network Address
Port Translation) devices, which modify the IP addresses and/or port
numbers. We anticipate that traversing such devices may require
variants of existing NAT/NAPT traversal mechanisms, e.g.,
encapsulation of the TCP-AO-protected segment in another transport
segment (e.g., UDP), as is done in IPsec [RFC2663][RFC3947]. Such
variants can be adapted for use with TCP-AO, or IPsec with NAT
traversal can be used instead of TCP-AO in such cases [RFC3947].
An alternate proposal for accommodating NATs extends TCP-AO
independently of this specification [To10].
10. Evaluation of Requirements Satisfaction
TCP-AO satisfies all the current requirements for a revision to TCP
MD5, as summarized below [Ed07].
1. Protected Elements
A solution to revising TCP MD5 should protect (authenticate) the
following elements.
This is supported -- see Section 5.1.
a. IP pseudoheader, including IPv4 and IPv6 versions.
Note that optional coverage is not allowed because IP addresses
define a connection. If they can be coordinated across a
NAT/NAPT, the sender can compute the MAC based on the received
values; if not, a tunnel is required, as noted in Section 9.2.
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RFC 5925 The TCP Authentication Option June 2010
b. TCP header.
Note that optional port coverage is not allowed because ports
define a connection. If they can be coordinated across a
NAT/NAPT, the sender can compute the MAC based on the received
values; if not, a tunnel is required, as noted in Section 9.2.
c. TCP options.
Note that TCP-AO allows the exclusion of TCP options from
coverage, to enable use with middleboxes that modify options
(except when they modify TCP-AO itself). See Section 9.
d. TCP payload data.
2. Option Structure Requirements
A solution to revising TCP MD5 should use an option with the
following structural requirements.
This is supported -- see Section 5.1.
a. Privacy.
The option should not unnecessarily expose information about
the TCP-AO mechanism. The additional protection afforded by
keeping this information private may be of little value, but
also helps keep the option size small.
TCP-AO exposes only the MKT IDs, MAC, and overall option length
on the wire. Note that short MACs could be obscured by using
longer option lengths but specifying a short MAC length (this
is equivalent to a different MAC algorithm, and is specified in
the MKT). See Section 2.2.
b. Allow optional per connection.
The option should not be required on every connection; it
should be optional on a per-connection basis.
This is supported because the set of MKTs can be installed to
match some connections and not others. Connections not
matching any MKT do not require TCP-AO. Further, incoming
segments with TCP-AO are not discarded solely because they
include the option, provided they do not match any MKT.
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RFC 5925 The TCP Authentication Option June 2010
c. Require non-optional.
The option should be able to be specified as required for a
given connection.
This is supported because the set of MKTs can be installed to
match some connections and not others. Connections matching
any MKT require TCP-AO.
d. Standard parsing.
The option should be easily parseable, i.e., without
conditional parsing, and follow the standard RFC 793 option
format.
This is supported -- see Section 2.2.
e. Compatible with Large Windows and SACK.
The option should be compatible with the use of the Large
Windows and SACK options.
This is supported -- see Section 7.6. The size of the option
is intended to allow use with Large Windows and SACK. See also
Section 1.3, which indicates that TCP-AO is 2 bytes shorter
than TCP MD5 in the default case, assuming a 96-bit MAC.
3. Cryptography requirements
A solution to revising TCP MD5 should support modern cryptography
capabilities.
a. Baseline defaults.
The option should have a default that is required in all
implementations.
TCP-AO uses a default required algorithm as specified in
[RFC5926] and as noted in Section 5.1 of this document.
b. Good algorithms.
The option should use algorithms considered accepted by the
security community, which are considered appropriately safe.
The use of non-standard or unpublished algorithms should be
avoided.
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RFC 5925 The TCP Authentication Option June 2010
TCP-AO uses MACs as indicated in [RFC5926]. The KDF is also
specified in [RFC5926]. The KDF input string follows the
typical design (see [RFC5926]).
c. Algorithm agility.
The option should support algorithms other than the default, to
allow agility over time.
TCP-AO allows any desired algorithm, subject to TCP option
space limitations, as noted in Section 2.2. The use of a set
of MKTs allows separate connections to use different
algorithms, both for the MAC and the KDF.
d. Order-independent processing.
The option should be processed independently of the proper
order, i.e., they should allow processing of TCP segments in
the order received, without requiring reordering. This avoids
the need for reordering prior to processing, and avoids the
impact of misordered segments on the option.
This is supported -- see Sections 7.3, 7.4, and 7.5. Note that
pre-TCP processing is further required, because TCP segments
cannot be discarded solely based on a combination of connection
state and out-of-window checks; many such segments, although
discarded, cause a host to respond with a replay of the last
valid ACK, e.g., [RFC793]. See also the derivation of the SNE,
which is reconstituted at the receiver using a demonstration
algorithm that avoids the need for reordering (in Section 6.2).
e. Security parameter changes require key changes.
The option should require that the MKT change whenever the
security parameters change. This avoids the need for
coordinating option state during a connection, which is typical
for TCP options. This also helps allow "bump in the stack"
implementations that are not integrated with endpoint TCP
implementations.
Parameters change only when a new MKT is used. See Section 3.
4. Keying requirements.
A solution to revising TCP MD5 should support manual keying, and
should support the use of an external automated key management
system (e.g., a protocol or other mechanism).
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RFC 5925 The TCP Authentication Option June 2010
Note that TCP-AO does not specify an MKT management system.
a. Intraconnection rekeying.
The option should support rekeying during a connection, to
avoid the impact of long-duration connections.
This is supported by the use of IDs and multiple MKTs; see
Section 3.
b. Efficient rekeying.
The option should support rekeying during a connection without
the need to expend undue computational resources. In
particular, the options should avoid the need to try multiple
keys on a given segment.
This is supported by the use of the KeyID. See Section 6.1.
c. Automated and manual keying.
The option should support both automated and manual keying.
The use of MKTs allows external automated and manual keying.
See Section 3. This capability is enhanced by the generation
of unique per-connection keys, which enables use of manual MKTs
with automatically generated traffic keys as noted in Section5.2.
d. Key management agnostic.
The option should not assume or require a particular key
management solution.
This is supported by use of a set of MKTs. See Section 3.
5. Expected Constraints
A solution to revising TCP MD5 should also abide by typical safe
security practices.
a. Silent failure.
Receipt of segments failing authentication must result in no
visible external action and must not modify internal state, and
those events should be logged.
This is supported - see Sections 7.3, 7.4, and 7.5.
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RFC 5925 The TCP Authentication Option June 2010
b. At most one such option per segment.
Only one authentication option can be permitted per segment.
This is supported by the protocol requirements - see Section2.2.
c. Outgoing all or none.
Segments out of a TCP connection are either all authenticated
or all not authenticated.
This is supported - see Section 7.4.
d. Incoming all checked.
Segments into a TCP connection are always checked to determine
whether their authentication should be present and valid.
This is supported - see Section 7.5.
e. Non-interaction with TCP MD5.
The use of this option for a given connection should not
preclude the use of TCP MD5, e.g., for legacy use, for other
connections.
This is supported - see Section 8.
f. "Hard" ICMP discard.
The option should allow certain ICMPs to be discarded, notably
Type 3 (destination unreachable), Codes 2-4 (transport protocol
unreachable, port unreachable, or fragmentation needed and IP
DF field set), i.e., the ones indicating the failure of the
endpoint to communicate.
This is supported - see Section 7.8.
g. Maintain TCP connection semantics, in which the socket pair
alone defines a TCP association and all its security
parameters.
This is supported - see Sections 3 and 9.
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RFC 5925 The TCP Authentication Option June 201011. Security Considerations
Use of TCP-AO, like the use of TCP MD5 or IPsec, will impact host
performance. Connections that are known to use TCP-AO can be
attacked by transmitting segments with invalid MACs. Attackers would
need to know only the TCP connection ID and TCP-AO Length value to
substantially impact the host's processing capacity. This is similar
to the susceptibility of IPsec to on-path attacks, where the IP
addresses and SPI would be visible. For IPsec, the entire SPI space
(32 bits) is arbitrary, whereas for routing protocols typically only
the source port (16 bits) is arbitrary (typically with less than 16
bits of randomness [La10]). As a result, it would be easier for an
off-path attacker to spoof a TCP-AO segment that could cause receiver
validation effort. However, we note that between Internet routers,
both ports could be arbitrary (i.e., determined a priori out of
band), which would constitute roughly the same off-path antispoofing
protection of an arbitrary SPI.
TCP-AO, like TCP MD5, may inhibit connectionless resets. Such resets
typically occur after peer crashes, either in response to new
connection attempts or when data is sent on stale connections; in
either case, the recovering endpoint may lack the connection key
required (e.g., if lost during the crash). This may result in
timeouts, rather than a more responsive recovery after such a crash.
Recommendations for mitigating this effect are discussed in Section7.7.
TCP-AO does not include a fast decline capability, e.g., where a SYN-
ACK is received without an expected TCP-AO and the connection is
quickly reset or aborted. Normal TCP operation will retry and
timeout, which is what should be expected when the intended receiver
is not capable of the TCP variant required anyway. Backoff is not
optimized because it would present an opportunity for attackers on
the wire to abort authenticated connection attempts by sending
spoofed SYN-ACKs without TCP-AO.
TCP-AO is intended to provide similar protections to IPsec, but is
not intended to replace the use of IPsec or IKE either for more
robust security or more sophisticated security management. TCP-AO is
intended to protect the TCP protocol itself from attacks that TLS,
sBGP/soBGP, and other data stream protection mechanisms cannot. Like
IPsec, TCP-AO does not address the overall issue of ICMP attacks on
TCP, but does limit the impact of ICMPs, as noted in Section 7.8.
TCP-AO includes the TCP connection ID (the socket pair) in the MAC
calculation. This prevents different concurrent connections using
the same MKT (for whatever reason) from potentially enabling a
traffic-crossing attack, in which segments to one socket pair are
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RFC 5925 The TCP Authentication Option June 2010
diverted to attack a different socket pair. When multiple
connections use the same MKT, it would be useful to know that
segments intended for one ID could not be (maliciously or otherwise)
modified in transit and end up being authenticated for the other ID.
That requirement would place an additional burden of uniqueness on
MKTs within endsystems, and potentially across endsystems. Although
the resulting attack is low probability, the protection afforded by
including the received ID warrants its inclusion in the MAC, and does
not unduly increase the MAC calculation or MKT management.
The use of any security algorithm can present an opportunity for a
CPU Denial-of-Service (DoS) attack, where the attacker sends false,
random segments that the receiver under attack expends substantial
CPU effort to reject. In IPsec, such attacks are reduced by the use
of a large Security Parameter Index (SPI) and Sequence Number fields
to partly validate segments before CPU cycles are invested validated
the Integrity Check Value (ICV). In TCP-AO, the socket pair performs
most of the function of IPsec's SPI, and IPsec's Sequence Number,
used to avoid replay attacks, isn't needed due to TCP's Sequence
Number, which is used to reorder received segments (provided the
sequence number doesn't wrap around, which is why TCP-AO adds the SNE
in Section 6.2). TCP already protects itself from replays of
authentic segment data as well as authentic explicit TCP control
(e.g., SYN, FIN, ACK bits) but even authentic replays could affect
TCP congestion control [Sa99]. TCP-AO does not protect TCP
congestion control from this last form of attack due to the
cumbersome nature of layering a windowed security sequence number
within TCP in addition to TCP's own sequence number; when such
protection is desired, users are encouraged to apply IPsec instead.
Further, it is not useful to validate TCP's Sequence Number before
performing a TCP-AO authentication calculation, because out-of-window
segments can still cause valid TCP protocol actions (e.g., ACK
retransmission) [RFC793]. It is similarly not useful to add a
separate Sequence Number field to TCP-AO, because doing so could
cause a change in TCP's behavior even when segments are valid.
12. IANA Considerations
The TCP Authentication Option (TCP-AO) was assigned TCP option 29 by
IANA action.
This document defines no new namespaces.
To specify MAC and KDF algorithms, TCP-AO refers to a separate
document [RFC5926].
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